Double Whammy: 2 Meteors Hit Ancient Earth at the Same Time

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It's not altogether uncommon to hear about double rainbows, but
what about a double meteor strike? It's a rare event, but
researchers in Sweden recently found evidence that two meteors
smacked into Earth at the same time, about 458 million years ago.

Researchers from the University of Gothenburg uncovered two
craters in the county of Jämtland in central Sweden. The
meteors that formed the craters landed just a few miles from
each other at the same moment, according to Erik Sturkell, a
professor of geophysics at the University of Gothenburg and one
of the scientists who is studying the newfound craters.

When the meteors slammed into Earth, Jämtland was just a
seafloor, about 1,600 feet (500 meters) below the surface of the
water. One of the craters left by the meteors is huge, measuring
4.7 miles (7.5 kilometers) across. The other, smaller crater —
which is only about 2,300 feet (700 m) across — is located just
10 miles (16 km) from its larger neighbor. [ Meteor
Crater: Experience an Ancient Impact ]

After analyzing information collected from a drilling operation,
the researchers determined that the impact
craters were formed at the same time. The information
revealed identical geological sequences, or layers of rock,
inside each crater. The sediment that accumulated inside the
craters over the subsequent millennia also dates back to the same
time, according to Sturkell.

"In other words, these are simultaneous impacts," Sturkell
said in a statement. The meteors likely crashed to Earth
following the collision of two large asteroids in the asteroid
belt between Mars and Jupiter some 470 million years ago, he
added.

When the
meteors crashed into Earth, they displaced the water
underneath them, leaving two huge, dry pits in the seabed for
about 100 seconds, the researchers said.

"The water then rushed back in, bringing with it fragments from
the meteorites mixed with material that had been ejected during
the explosion and with the gigantic wave that tore away parts of
the seabed," Sturkell said.

This isn't the first time that scientists in the area have found
evidence of ancient meteor impacts in what is now Sweden, though
it is the first time they've found evidence of two meteors
striking the planet at the same moment.

In the 1940s, quarry workers found an unusual red slab of
limestone on Kinnekulle, a large hill in the county of
Västergötland in southern Sweden. Researchers later identified
the red rock as a meteorite. While
large meteorites typically "explode and disintegrate" upon
impact with the ground, small meteors fall to Earth as rocks,
like the one embedded in the limestone slab, Sturkell said.

About 90 meteorites in total have been found on Kinnekulle in the
past 15 years alone. But in Jämtland, where the dual meteor
strikes occurred, researchers have only found small grains of
chromite, a remnant of large, exploded meteors.